You're Getting Warm if You're Thinking Cool

Janet J. Barron

You won't be working at a cryogenic workstation this year or next. But before the turn of the century, you may be. Recent advances in the field of cryoelectronics, which involves computing at very low temperatures, mean scientists can now evaluate superconductivity at the relatively high temperature range of 77 K (i.e., -356 degrees F) to 135 K. Previous experiments in this area involved working at temperatures in the range of 10 K to 15 K, or about -443 degrees F. By the year 2000, systems could be available that use the following types of HTS (high-temperature superconductive) devices: semiconductors; interconnections between processors, chips, and boards; DRAM chips; and even MCMs (multichip modules).

The benefits of these HTS sys tems include increases in speed (10 to 100 times), reductions in power (90 percent or more) and reliability (due to their operation with little resistance). According to Joseph Warner, materials electronics engineer at the NASA Lewis Research Center (Cleveland, OH), a refrigerator the size of a Coca-Cola can that can cool these HTS components has already been developed for the space effort.

Scientists have produced successful experimental (and some prototypical) HTS devices for signal processing, ADCs (A/D converters), optical sensors, logic, and memory. Also, analog HTS arrays have been developed for microwave communications and telemetry applications.

The U.S. government's ARPA has a program to develop HTS devices. Among the 20 participating firms are Superconductor Technologies, Inc. (Santa Barbara, CA), Conductus (Sunnyvale, CA), and E-Systems (Dallas, TX). Joseph M. Madden, manager of applications engineering for STI, is one of many who believe that an HTS computing environment is a must.

"HTS interconnects, for example," Madden says, "have demonstrated the ability to pass signals with much greater speed than normal metal interconnects." Madden says that tests performed at STI on a 10-centimeter, 3-micron copper line and an HTS transmission line showed that the HTS line can operate at least twice as fast as an identical copper line.

Claude Hilbert, a member of the technical staff at Microelectronics and Computer Technology (Austin, TX), believes that the gap between microprocessor and memory is where we need HTS technology. He is close to assembling and testing a second-generation HTS hybrid memory (i.e., fast RAM). The ultimate goal of his project is to create a monolithic hybrid chip--a chip with both HTS and semiconductor devices--to be used in high-performance cryogenic workstations.

As if it were not tough enough to develop HTS chips, a number of researchers are working on an even more challenging undertaking--creating an HTS MCM. A popular packaging technology, an MCM provides shorter interconnections, high chip densities, and improved IC system performance. HTS MCMs would magnify these benefits.

During the first quarter of 1993, Conductus and STI scientists demonstrated that it is possible to produce a hybrid structure combining active semiconductor and HTS devices on the same substrate. They believe that the successful integration of these disparate yet complementary technologies paves the way for the development of true semiconductive-superconductive hybrid ICs capable of combining the best of both worlds.

Although the benefits of HTS are eagerly awaited by many, not everyone sees HTS devices as tomorrow's technology of choice. The technology has many challenges to overcome. Some computer manufacturers are skeptical that successful solutions will be reached in a timely enough manner to bring relief for some of the difficulties facing the industry. Others have decided to continue using conventional semiconductor devices because of their already outstanding capabilities.

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Illustration: Typical HTS MCM In HTS MCMs, line widths in transmission lines can be very narrow (i.e., 2 microns), allowing a higher packing density and improved signal transmission over conventional MCMs. Shown here is the module that wires multiple chips together while providing their ground plane and power source.